Did you guys need to add a capacitor when you put driving lights on your jeep? I was looking at putting 2 6" lights on the windshield mounts but I recently added a JL audio 10" sub and its only 150 watts RMS and it causes my headlights to dim

 

Its the amp, not the sub that causes the headlights to dim. Make sure you have a solid ground, and a good connection at the battery. 150w shouldn't make the lights dim, I'm running much more power with no dimming at all.

 

The sort of capacitor your thinking of is for maintaining voltage during a big draw from your amp. Headlights are a steady load and will not drive the need for a capacitor on the line.
I have a 400W amp in my little Corolla and it will dim the lights a bit, but 150 watts should not dim them.